1. Field of the Invention
The present invention relates to methods for producing a high pressure discharge lamp, and to high pressure discharge lamps and lamp units. In particular, the present invention relates to methods for producing a high pressure discharge lamp used for general illumination, a projector or an automobile headlight in combination with a reflecting mirror, or the like.
2. Description of the Related Art
In recent years, an image projecting apparatus such as a liquid crystal projector and a DMD (Digital Micromirror Device) projector has been commonly used as a system for realizing large-scale video images, and in general, a high pressure discharge lamp having a high intensity has been commonly used for such an image projecting apparatus. FIG. 11 is a schematic view showing the structure of a conventional high pressure discharge lamp 1000. The lamp 1000 shown in FIG. 11 is a so-called ultrahigh pressure mercury lamp, which is disclosed, for example, in Japanese Unexamined Patent Publication No. 2-148561.
The lamp 1000 includes a luminous bulb (arc tube) 101 made of quartz glass and a pair of sealing portions (seal portions) 102 extending from both ends of the luminous bulb 101. A luminous material (mercury) 106 is enclosed inside (in a discharge space) of the luminous bulb 101, and a pair of tungsten electrodes (W electrodes) 103 made of tungsten are opposed with a predetermined distance. A molybdenum foil (Mo foil) 104 in the sealing portion 102 is welded to one end of the W electrode 103, and the W electrode 103 and the Mo foil 104 are electrically connected to each other. An external lead (Mo rod) 105 made of molybdenum is electrically connected to one end of the Mo foil 104. Argon (Ar) and a small amount of halogen, in addition to the mercury 106, are enclosed in the luminous bulb 101.
The operational principle of the lamp 1000 will be briefly described below. When a start voltage is applied between the W electrodes 103 via the external leads 105 and the Mo foils 104, discharge of argon (Ar) occurs. This discharge increases the temperature in the discharge space of the luminous bulb 101, and then the mercury 106 is heated and evaporated. Therefore, mercury atoms are exited in the central portion of an arc between the W electrodes 103 and thus light is emitted. The higher the mercury vapor pressure of the lamp 1000 is, the more light is radiated, so that the lamp with a higher mercury vapor pressure is more suitable for the light source of an image projecting apparatus. However, in view of the physical strength of the luminous bulb 110 against pressure, the lamp 1000 is used at a mercury vapor pressure of 15 to 20 MPa (150 to 200 atm).